1. Field of the Invention
The invention relates generally to the field of direct-to-digital holography (interferometry). More particularly, the invention relates to off-axis illumination for improved resolution in direct-to-digital holography.
2. Discussion of the Related Art
Prior art direct-to-digital holography (DDH), sometimes called direct-to-digital interferometry, is known to those skilled in the art. For instance, FIG. 1 illustrates one simplified embodiment of a DDH system. Light from a laser source 105 is expanded by a beam expander/spatial filter 110 and then travels through a lens 115. Subsequently, the expanded filtered light travels to a beamsplitter 120. The beamsplitter 120 may be partially reflective. The portion of light reflected from the beamsplitter 120 constitutes an object beam 125 which travels to the object 130. The portion of the object beam 125 is that is reflected by the object 130 then passes through the beamsplitter 120 and travels to a focusing lens 145. This light then passes through the focusing lens 145 and travels to a charge coupled device (CCD) camera (not shown).
The portion of the light from the lens 115 that passes through the beamsplitter 120 constitutes a reference beam 135. The reference beam 135 is reflected from a mirror 140 at a small angle. The reflected reference beam 135 from the mirror then travels toward the beamsplitter 120. The portion of the reference beam 135 that is reflected from the beamsplitter 120 then travels through the focusing lens 145 and toward the CCD camera (not shown). The object beam 125 from the focusing lens 145 and the reference beam 135 from the focusing lens 145 constitute a plurality of object and reference waves 150 and will interfere at the CCD to produce the interference pattern characteristic of a hologram as noted in U.S. Pat. No. 6,078,392.
In FIG. 1, the object beam 125 is parallel to, and coincident with, the optical axis 127. This type of DDH set-up can be referred to as on-axis illumination.
A limitation of this technology has been that the imaging resolution of the DDH system is limited by the optics of the system. The most notable limitation of the optics is the aperture stop, which is required to prevent degradation of the image quality due to aberrations. With regard to a two-dimensional Fourier plane, only object spatial frequencies within a circle of radius q0 can be transmitted. In the case of on-axis illumination, the aperture with radius q0 appears centered on a zero spatial frequency (q=0). What is needed, therefore, is an approach that permits spatial frequencies outside the circle of radius q0 to be transmitted.
There is a need for the following aspects of the invention. Of course, the invention is not limited to these aspects.
According to an aspect of the invention, a process of recording an off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprises, reflecting a reference beam from a reference mirror at a non-normal angle; reflecting an object beam from an object at an angle with respect to an optical axis defined by a focusing lens; focusing the reference beam and the object beam at a focal plane of a digital recorder to form the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; digitally recording the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; Fourier analyzing the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes by transforming axes of the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined as an angle between the reference beam and the object beam; applying a digital filter to cut off signals around an original origin; and then performing an inverse Fourier transform.
According to another aspect of the invention, a machine operable to digitally record an off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprises: a laser; a beamsplitter optically coupled to the laser; a reference beam mirror optically coupled to the beamsplitter; a focusing lens optically coupled to the reference beam mirror; a digital recorder optically coupled to the focusing lens; and a computer that performs a Fourier transform, applies a digital filter, and performs an inverse Fourier transform, wherein a reference beam is incident upon the reference beam mirror at a non-normal angle, an object beam is incident upon an object at an angle with respect to an optical axis defined by the focusing lens, the reference beam and the object beam are focused by the focusing lens at a focal plane of the digital recorder to form the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis which is recorded by the digital recorder, and the computer transforms axes of the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined by an angle between the reference beam and the object beam and cuts off signals around an original origin before performing the inverse Fourier transform.
These, and other, aspects of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.